US investors in Vodafone are being targeted by rebel shareholders who want to force the mobile phone company to spin off its US business and increase its debts to return more cash to investors.

With Vodafone's annual meeting set for next Tuesday, the activist shareholder Efficient Capital Structures (ECS) has welcomed backing from American investor group Glass Lewis for its proposals. About 30% of Vodafone shares are US-held and ECS - backed by former Marconi executive John Mayo and Glenn Cooper, the City banker who floated Manchester United - has been focusing on investors in America to back its four resolutions.

Glass Lewis, which advises 80% of US funds, has recommended that Vodafone shareholders support all but one of those - rejecting ECS's suggestion that any deal over £1bn require shareholder approval. The group has also advised investors to reject Vodafone's remuneration report.

Raising the prospect of a fresh row over directors' pay, Glass Lewis tells investors it has "severe reservations" about approving the company's remuneration policy.

At last year's annual meeting, investors expressed their anger at a decision to relax targets on the executive bonus scheme, with more than 20% who voted refusing to support the policy. For this year, Glass Lewis flagged up the company's move to soften performance targets once again and raised concerns it may "signal that the remuneration committee will now allow executives to be excessively rewarded for less-than-superior performance".

ECS, wants the company to release up to £38bn by handing its 45% stake in American joint venture Verizon Wireless back to shareholders and increasing its debts in order to return cash to investors. Vodafone has rejected this idea - which it has considered in the past - as undermining its ability to maximise the value of the Verizon Wireless holding. It also reckons that increasing its debt by £34bn through a massive bond issue would hinder its ability to invest in its businesses.

The Vodafone board has already received the backing of several key investors including M&G Investment Management, Insight and Morley Fund Management. But it has a diverse ownership base with 2,500 institutional shareholders owning 98% of the business.

A large portion of US investors will have to lodge their votes for this year's annual meeting by the end of this week. While ECS has welcomed Glass Lewis's support for most of its resolutions, it has yet to receive any public backing from shareholders in the way Vodafone has. Wide support is crucial because it needs 50% of investors to vote in favour of some of its resolutions and as much as 75% on others.

This month, the Association of British Insurers refused to back the board of Vodafone in its battle against the rebel shareholders. The ABI, whose members speak for 20% of the shares listed on the London Stock Exchange, issued what it termed an "amber top" alert, intended to draw its members' attention to the resolutions that will be put to the meeting. But it has backed away from telling members whether to vote for or against the four resolutions tabled by ECS.

However, Research, Recommendations and Electronic Voting (RREV) has advised shareholders to vote against the Mayo resolutions. RREV is partly owned by the National Association of Pension Funds, which accounts for £600bn of pension fund assets.